New efficiency record advances spray-on solar cells

Advancing solar technology is a trade-off between the efficiency of the cells themselves and the cost of producing and installing them. Quantum dot solar cells, which use nanoscale semiconductors to produce electricity, promise low-cost production and, because they can be sprayed or painted on, big benefits in terms of installation. In the efficiency stakes quantum cells don't score as well as silicon-based or CIGS solar cells, but a new efficiency record for colloidal quantum dot solar cells represents a big step towards narrowing the gap. This breakthrough isn't about the quantum dots though, it's about the wrapping.

Colloidal quantum dot solar cells

To make quantum dot solar cells that can be sprayed or painted on, the tiny nanoscale semiconductors need to be dispersed evenly within another substance - this is known as a colloid. Organic molecules have successfully been used to create colloidal quantum dot (CQD) solar cells but the space they take up between the nanoparticles (and we're only talking one or two nanometers here) has a negative impact on the flow of electrons and in turn the efficiency of the cell.

In order to make the world's most efficient colloidal quantum dot solar cell, researchers from the University of Toronto, King Abdullah University of Science & Technology and Pennsylvania State University developed a way to coat or passivate the quantum dots in inorganic ligands. This approach improves efficiency by reducing electron "traps" in the material and packing more quantum dots into the same amount of nanospace, while at the same time retaining the colloid characteristics of quantum dots bound together by organic molecules.

"We wrapped a single layer of atoms around each particle," explains Dr. Jiang Tang, the first author of the paper. "As a result, they packed the quantum dots into a very dense solid."

New efficiency record

The result is a colloidal quantum dot solar cell with a record efficiency of 6 percent. While current silicon solar cells run in the 14 percent efficiency ballpark, the 6 percent figure coupled with the inherent benefits of quantum dot cells (and the added bonus that inorganics can be manufactured at room temperature) makes this a significant development.

The record - which included both the highest electrical currents and the highest overall power conversion efficiency for CQD solar cells - was certified by an external laboratory accredited by the US National Renewable Energy Laboratory.

The University of Toronto and King Abdullah University of Science & Technology have signed a licensing agreement with a view to commercializing the technology, which also has potential applications beyond the solar realm in fields like optoelectronics.

"The world - and the marketplace - need solar innovations that break the existing compromise between performance and cost. Through U of T's, MI's, and KAUST's partnership, we are poised to translate exciting research into tangible innovations that can be commercialized," said Professor Ted Sargent, corresponding author on the work and holder of the Canada Research Chair in Nanotechnology at U of T.

The researchers outline their work in the paper Collodial-quantum-dot photovoltaics using atomic-ligand passivation and the breakthrough is reported in the latest issue of Nature Materials.